In a following patch, on finding a free area we will need to do
allocatinon of extra contexts as needed. Consolidating the return path
for slice_get_unmapped_area() will make that easier.

Split into a separate patch to make review easy.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>